The invention relates to a semiconductor device having a semiconductor body which comprises at least one semiconductor element with an active region and a coil, which coil is coupled to the semiconductor element and forms part of a transformer which includes a further coil, which semiconductor body is mounted onto a carrier plate which comprises an electrically insulating material and is provided with a conductor track.
Such a device, comprising, for example, a bipolar transistor as the semiconductor element, is used, inter alia, as an amplifier of electronic signals, in particular as a high-frequency power amplifier.
Such a device is disclosed in U.S. Pat. No. 5,519,582, published on May 21, 1996. In said patent specification (see FIG. 1), a description is given of a semiconductor device comprising a semiconductor body wherein an active semiconductor element is situated on the upper side, and the lower side of said semiconductor body is provided with a trench filled with a conductor, which trench forms a coil and is coupled to the semiconductor element. The semiconductor body is mounted on a carrier plate provided with a conductor track. Said carrier plate is, for example, a PCB (=Printed Circuit Board) or a ceramic substrate. The coil may form part of a transformer. An intended application of such a device is as a stable power supply.
A drawback of the known device resides in that the manufacture thereof is relatively complex because the semiconductor body must be subjected to many process steps on both sides. The integration of a transformer, and hence a further coil, in the semiconductor body renders the manufacture even more complicated and, in addition, a comparatively strong capacitive coupling between both coils occurs because the coils must be intertwined.
Therefore, it is an object of the invention to provide a device in which the above drawbacks are completely, or at least substantially, obviated, which device is easy to manufacture, and the transformer employed therein exhibits a small capacitive coupling between the coils.
To achieve this, a device of the type mentioned in the opening paragraph is characterized in accordance with the invention in that said further coil is situated on the carrier plate, forms part of the conductor track and is electrically separated from the coil. By positioning the further coil on the carrier plate, the manufacture of the device is simplified to a substantial degree because the semiconductor body has to be subjected to a larger number of process steps on one side only. In the formation of the further coil, a further simplification is obtained by using the conductor track which is situated on the carrier plate and which is necessary anyway. The invention is further based on the recognition that the two coils necessary for a transformer do not necessarily have to be either both integrated in the semiconductor body or both situated outside the semiconductor body. In fact, a device in accordance with the invention has an important additional advantage, partly as a result of the fact that both coils are electrically separated, the additional advantage being that the signal transfer from the external world to the semiconductor body now can take place in a contact-free manner, namely via the magnetic coupling of both coils instead of, for example, via electric connection wires.
In a preferred embodiment, the coil is situated on the same side of the semiconductor body as the active region of the semiconductor element. By virtue thereof, both the coil and the semiconductor element can be manufactured (for the most part) during the same process steps. For example, the conductor necessary to form the coil can also be used as the connection conductor for the semiconductor element.
Preferably, the active region of the semiconductor element and the coil are situated on one side of the semiconductor body which is secured with the other side to the carrier plate. In this modification, it is still possible to bring about a satisfactory magnetic coupling between the coil and the further coil. For this purpose, the diameter of the coils must be larger than the distance between the two coils. This distance is equal to the thickness of the semiconductor body and amounts to, for example, 500 xcexcm in practice, while the diameter of the coil often amounts to, for example, 600 xcexcm in practice.
In a favorable further modification, the semiconductor body is embodied so as to be thinner or interrupted at the location of the coil. This results in a particularly low parasitic capacitance and a good magnetic coupling because the magnetic lines extend through the semiconductor material, for example silicon.
A very attractive modification of a device in accordance with the invention, wherein the semiconductor element comprises a bipolar transistor having, a base, an emitter, and a collector, is characterized in that the semiconductor body is secured, on one side, to an insulating substrate by means of an adhesive layer, which semiconductor body comprises at least three mesa-shaped parts which include, respectively, the active region of the transistor and a collector-connection region, a base-connection region and an emitter-connection region, and by means of which mesa-shaped parts the semiconductor body is secured, on the other side, to a further conductor track on the carrier plate. In this modification, the thickness of the semiconductor body can be relatively small because the semiconductor body is secured to an insulating substrate. By virtue thereof, also the coil can have a relatively small diameter, without the magnetic coupling being adversely affected. In this case, the device is also very compact. In addition, the device can be readily manufactured by virtue of the SMD (=Surface Mounted Device) technique used to secure the semiconductor body on the carrier plate.
A method of manufacturing a semiconductor device comprising a semiconductor body wherein at least one semiconductor element with an active region is formed as well as a coil which is coupled to said semiconductor element, which coil forms part of a transformer which comprises a further coil, the semiconductor body being secured on a carrier plate which comprises an electrically insulating material and is provided with a conductor track, is characterized in accordance with the invention in that the further coil is provided on the carrier plate and formed in the conductor track and electrically separated from the coil. In this manner, a device in accordance with the invention is obtained in a simple manner.
In a favorable modification, the active region of the semiconductor element and the coil are formed on the same side of the semiconductor body. Preferably, the active region of the semiconductor element and the coil are formed on one side of the semiconductor body, and the semiconductor body is secured on the carrier plate on the other side. In a further favorable modification, the semiconductor body is embodied so as to be thinner or interrupted at the location of the coil.
In a preferred embodiment, wherein a bipolar transistor having an emitter, a base and a collector is formed as the semiconductor element, after the formation of the transistor and the coil on one side of the semiconductor body, said semiconductor body is glued onto an insulating substrate on this side, whereafter, on the other side of the semiconductor body, at least three mesa-shaped parts are formed from said semiconductor body by locally removing the semiconductor material of the semiconductor body, one of said mesa-shaped parts comprising the active region of the transistor and a collector-connection region, another mesa-shaped part comprising a base-connection region, and a further mesa-shaped part comprising an emitter-connection region, whereafter the semiconductor body is secured to a further conductor track on the carrier plate by means of said mesa-shaped parts.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.